xfs: create a helper to count per-device inode block usage

Create a helper to compute the number of blocks that a file has
allocated from the data realtime volumes.  This patch was
split out to reduce the size of the upcoming quotacheck patch.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>